1. Field of the Invention
The invention relates to the vertical stacking of integrated circuits to increase the density of components on a printed circuit board without increased footprint. More particularly, the present invention relates to apparatus and methods for the vertical stacking of memory integrated circuits on a surface mount printed circuit board.
2. Description of the Related Art
Modern electronic devices, such as computers and the like, typically include integrated circuits commonly referred to and will be referred to herein as “chips”. Integrated circuits or chips are microcircuits formed on a semiconductor substrate and packaged in a ceramic, plastic or epoxy package having multiple external terminals or “pins”. The microcircuits are wire-bonded within the package to the external terminals or pins. When the pins of the chip packages are connected to the printed circuit board, the integrated circuits are electrically connected to other integrated circuits and electrical components through or by way of traces on the printed circuit board to form system level electronic circuits.
With advances in semiconductor device processing has come a continuing increase in device count and density within chips and this has driven a corresponding increase in the count and density of the external conducting pads. Current technology places a limit on how small external contacts can be made and how closely they can be placed adjacent one another while still maintaining circuit integrity. Limits are imposed, both by the limitations of machinery to form ever-smaller conductive elements and by the reduction in production yield as the limits are pushed.
Additionally, as modern electronic devices are driven to ever increasing functionality and decreasing size, the printed circuit boards within the electronic devices, are driven to increased integrated circuit densities. The desire to provide the capability of integrated circuits to be used in relatively small devices limits the extent to which multiple chips can be laterally interconnected while still fitting within the device. Lateral extension and interconnection of chips tends to lead to relatively long interconnects or traces between chips which increases the signal propagation delay and thus, decreases the circuit operating speed. Further, lengthy traces increase both the radio-frequency interference (RFI), and electromagnetic interference (EMI) emitted from the printed circuit board.
From the foregoing, it can be appreciated that there is an ongoing need for structures and methods for interconnecting chips that increase circuit density without increasing the chip footprint and with minimal increase in interconnection length.